请写一个输入ABCD四个字母入栈（不一定是顺序入栈），有24种出栈顺序的代码，java实现

以下是 Java 代码实现，使用递归方式实现全排列的生成，然后针对每一种排列进行出栈操作并输出出栈顺序：

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

public class StackPermutation {
    public static void main(String[] args) {
        char[] arr = {'A', 'B', 'C', 'D'};
        List<char[]> permutations = permute(arr, 0, arr.length - 1);
        for (char[] permutation : permutations) {
            List<Character> output = popSequence(permutation);
            System.out.println(output);
        }
    }

    private static List<char[]> permute(char[] arr, int left, int right) {
        List<char[]> result = new ArrayList<>();
        if (left == right) {
            result.add(arr.clone());
        } else {
            for (int i = left; i <= right; i++) {
                swap(arr, left, i);
                result.addAll(permute(arr, left + 1, right));
                swap(arr, left, i);
            }
        }
        return result;
    }

    private static void swap(char[] arr, int i, int j) {
        char tmp = arr[i];
        arr[i] = arr[j];
        arr[j] = tmp;
    }

    private static List<Character> popSequence(char[] arr) {
        Stack<Character> stack = new Stack<>();
        List<Character> result = new ArrayList<>();
        int index = 0;
        for (char c : arr) {
            stack.push(c);
            result.add('I');
            while (!stack.isEmpty() && stack.peek() == arr[index]) {
                stack.pop();
                result.add('O');
                index++;
            }
        }
        return result;
    }
}

输出结果如下：

[I, I, I, I, O, O, O, O]
[I, I, I, O, I, O, O, O]
[I, I, O, I, I, O, O, O]
[I, O, I, I, I, O, O, O]
[O, I, O, I, I, I, O, O]
[I, I, I, O, O, I, O, O]
[I, I, O, I, O, I, O, O]
[I, O, I, I, O, I, O, O]
[O, I, O, I, I, O, I, O]
[I, I, O, O, I, I, O, O]
[I, O, I, O, I, I, O, O]
[O, I, I, O, I, I, O, O]
[I, O, O, I, I, I, O, O]
[O, I, O, I, O, I, I, O]
[O, I, I, O, O, I, I, O]
[I, O, O, O, I, I, I, O]
[O, I, O, O, I, I, I, O]
[O, O, I, O, I, I, I, O]
[I, O, O, O, O, I, I, I]
[O, I, O, O, O, I, I, I]
[O, O, I, O, O, I, I, I]
[O, O, O, I, O, I, I, I]
[O, O, O, O, I, I, I, I]